Sheet feeder and image forming apparatus

ABSTRACT

A sheet feeder includes a spool and a pair of roll holders. The spool is configured to hold a roll of print medium having an adhesive surface with no release liner attached thereto, with the roll fitted around the spool. The pair of roll holders is configured to hold the spool at opposed ends of the spool, each including at least a first rotary member, a second rotary member, and a third rotary member that contact an outer circumferential surface of the spool at different locations along the outer circumferential surface of the spool to rotatably hold the spool.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2012-224482, filed on Oct. 9, 2012, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein.

BACKGROUND

1. Technical Field

The present invention relates to a sheet feeder and an image forming apparatus, particularly to a sheet feeder and an image forming apparatus using a rolled print medium having an adhesive surface.

2. Related Art

An image forming apparatus, such as a label printer, performs printing on a print medium having an adhesive surface with no release liner attached thereto, such as an adhesive tape or a label sheet with no backing sheet, (hereinafter also referred to as a linerless label sheet), and thereafter cuts the print medium into print medium pieces (hereinafter also referred to as label pieces) of a desired length.

A sheet transporting apparatus or image forming apparatus using a rolled sheet may include, for example, a spool, a transporting device, and a rotational load device. The spool holds a rolled sheet and is rotatable in both a sheet feeding direction and a sheet rewinding direction. The transporting device transports the sheet through an image forming area. The rotational load device places a predetermined rotational load on the spool. Drive force in the sheet feeding direction is not transmitted to the rotational load device, and drive force in the sheet rewinding direction is transmitted to the rotational load device.

If the rolled sheet used in the apparatus is a roll of linerless label sheet having an adhesive surface, then when the linerless label sheet is unwound from the roll to be fed and transported, a force for peeling the linerless label sheet off the roll (also referred to as peel force) is generated in a sheet feeder of the apparatus. When the linerless label sheet is transported to an image forming device of the apparatus to form an image on the linerless label sheet, therefore, the peel force generated in the sheet feeder affects and changes the transport speed and the transport distance, degrading the transport accuracy.

SUMMARY

It is an object of the present invention to improve the accuracy of transporting a linerless label sheet. The present invention provides an improved sheet feeder that, in one example, includes a spool and a pair of roll holders. The spool is configured to hold a roll of print medium having an adhesive surface with no release liner attached thereto, with the roll fitted around the spool. The pair of roll holders is configured to hold the spool at opposed ends of the spool, each including at least a first rotary member, a second rotary member, and a third rotary member that contact an outer circumferential surface of the spool at different locations along the outer circumferential surface of the spool to rotatably hold the spool.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the advantages thereof are obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a front view of a mechanical section of an image forming apparatus according to an embodiment of the present invention;

FIG. 2 is a front view of a transporting unit of the mechanical section;

FIG. 3 is a front view of related parts of the transporting unit transporting a print medium;

FIG. 4 is a perspective view of a sheet feeding unit according to the embodiment, in which a roll is installed;

FIG. 5 is a perspective view of the sheet feeding unit, from which the roll is removed;

FIG. 6 is a front view of the sheet feeding unit with the roll installed;

FIG. 7 is a front view of the sheet feeding unit with an openably closable member thereof opened;

FIG. 8 is a front view of the sheet feeding unit and the roll removed therefrom; and

FIG. 9 is a front view of the sheet feeding unit, in which a spool holding the roll receives drive force from a drive source of the image forming apparatus via a drive force transmission mechanism.

DETAILED DESCRIPTION

In describing the embodiments illustrated in the drawings, specific terminology is adopted for the purpose of clarity. However, the disclosure of the present invention is not intended to be limited to the specific terminology so used, and it is to be understood that substitutions for each specific element can include any technical equivalents that have the same function, operate in a similar manner, and achieve a similar result.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, an embodiment of the present invention will be described.

With reference to FIGS. 1 to 3, an image forming apparatus according to an embodiment of the present invention will be described. FIG. 1 is a front view of a mechanical section of the image forming apparatus. FIG. 2 is a front view of a transporting unit of the mechanical section. FIG. 3 is a front view of related parts of the transporting unit transporting a print medium.

The image forming apparatus includes an apparatus body 100 including a sheet feeding unit 101 (i.e., a sheet feeder), an image forming unit 102 (i.e., an image forming device), a transporting unit 103 (i.e., a transporting device), a sheet discharging unit 104 (i.e., a sheet discharging device), a discharge opening 105, a guiding device 106, ink cartridges 15, and a waste liquid tank 17. The guiding device 106 guides a print medium 2 (also referred to as a recording medium or a sheet, for example) during transport or rewinding of the print medium 2.

The print medium 2 is wound in a roll 4, which is installed in the sheet feeding unit 101. As illustrated in FIG. 3, in the present embodiment, the print medium 2 is a continuum of an image-formable medium having a surface formed with an adhesive layer. Hereinafter, the image-formable medium and the adhesive layer will be referred to as the printing surface 2 a and the adhesive surface 2 b, respectively. Specifically, the print medium 2 is a rolled linerless label sheet with no backing sheet (i.e., release liner or separator) attached to the adhesive surface 2 b.

The sheet feeding unit 101 includes a spool 5 and two opposed roll holders 6. FIGS. 1 and 2 illustrate one of the two roll holders 6, i.e., the roll holder 6 on the front side of the apparatus body 100. As illustrated in FIG. 2, each of the roll holders 6 includes a spool bearing 7, an openably closable member 8, a first roller 111, a second roller 112, a third roller 113, a shaft 9, a shaft 114, a roller holder 115, and a tension spring 116 described in detail later.

The image forming unit 102 includes a carriage 12, two recording heads 11, a main guide member 13, a sub-guide member 14, and supply tubes 16. The recording heads 11, which are liquid ejection heads that eject liquid droplets onto the print medium 2, are mounted on the carriage 12. The carriage 12 is movably held by the main guide member 13 and the sub-guide member 14 to move from side to side in a main scanning direction substantially perpendicular to the direction of transporting the print medium 2 (hereinafter referred to as the medium transporting direction).

The present embodiment uses, as the recording heads 11, two liquid ejection heads each including two nozzle rows (not illustrated) to eject ink droplets of black, cyan, magenta, and yellow colors from four nozzle rows. The recording heads 11, however, are not limited thereto, and may be line heads. Inks of the respective colors are supplied as necessary from the ink cartridges 15 replaceably installed in the apparatus body 100 to head tanks (not illustrated) of the carriage 12 through the supply tubes 16, and then to the recording heads 11. Waste ink resulting from, for example, a maintenance operation for maintaining and restoring the performance of the recording heads 11 is discharged to and stored in the waste liquid tank 17 replaceably installed in the apparatus body 100. In the image forming unit 102, the form of the recording heads 11 (i.e., liquid ejection heads) is not limited, as described above. Further, various types of image forming devices that form an image on a print medium in a contact or non-contact manner are applicable to the image forming unit 102.

The transporting unit 103 includes an endless protection belt 21, a transport roller 22, a driven roller 23, a facing roller 24, a suction fan 27, and spur roller groups 28 a, 28 b, and 28 c. The protection belt 21 serving as a transport belt is disposed below the recording heads 11, and is rotatably stretched taut around the transport roller 22 and the driven roller 23. Preferably, the protection belt 21 is not adhesive to the adhesive surface 2 b of the print medium 2. The protection belt 21, however, may exhibit weak adhesion to the adhesive surface 2 b of the print medium 2 to prevent the print medium 2 from separating from the protection belt 21 during the transport of the print medium 2 (hereinafter referred to as the medium transporting operation) and separate from the print medium 2 after the medium transporting operation. Received by the protection belt 21, the adhesive surface 2 b of the print medium 2 is protected and prevented from coming into contact with other components inside the apparatus body 100, thereby providing stable transport performance. Further, due to the separability of the protection belt 21 from the adhesive surface 2 b of the print medium 2, the print medium 2 is reliably sent to the next process. That is, the protection belt 21 of the present embodiment functions as a transport belt and also as a protector of the adhesive surface 2 b of the print medium 2.

The facing roller 24 is disposed facing the transport roller 22. The transport roller 22 and the facing roller 24 form a transport roller pair (i.e., a rotary member pair) serving as a transporting device that clamps and transports the print medium 2 and the protection belt 21 to an image forming area in which an image is formed by the recording heads 11. The protection belt 21 is formed with a multitude of suction holes therein. Inside a loop of the protection belt 21, the suction fan 27 is disposed facing the recording heads 11 of the image forming unit 102 via the protection belt 21. The suction fan 27 sucks the print medium 2 toward the outer circumferential surface of the protection belt 21 through the suction holes.

It is to be noted that the transporting unit 103 of the present embodiment is configured to attract the print medium 2 toward the protection belt 21 by suction. However, the configuration is not limited thereto. For example, the transporting unit 103 may be configured to attract the print medium 2 electrostatically, Alternatively, the print medium 2 may be kept in contact with the protection belt 21 not to separate therefrom by the adhesion of the adhesive surface 2 b of the print medium 2.

The spur roller groups 28 a, 28 b. and 28 c are disposed near the driven roller 23 each of which includes a plurality of spur rollers aligned in a direction substantially perpendicular to the medium transporting direction. The spur roller groups 28 a and 28 b located on the upstream side in the medium transporting direction face the protection belt 21, and the most downstream spur roller group 28 c faces a receiving member 30 of the sheet discharging unit 104.

The sheet discharging unit 104 includes the receiving member 30, a cutter unit 31, a discharge roller 32, a spur roller group 33, and a sheet sensor 34. The receiving member 30 guides the print medium 2 sent thereto from between the protection belt 21 and the spur roller group 28 b. The cutter unit 31 disposed downstream of the receiving member 30 in the medium transporting direction serves as a cutting device that cuts the print medium 2 into print medium pieces, i.e., label pieces 200 of a desired length. The cutter unit 31 includes an upper cutter 313 and a lower cutter formed by a downstream end surface of the receiving member 30 receiving the print medium 2. The upper cutter 313 moves in a direction substantially perpendicular to the medium transporting direction to cut the print medium 2 in conjunction with the lower cutter.

The discharge roller 32 is disposed downstream of the cutter unit 31 in the medium transporting direction to face the spur roller group 33 including a plurality of spur rollers aligned in a direction substantially perpendicular to the medium transporting direction. The discharge roller 32 and the spur roller group 33 hold the label piece 200 cut by the cutter unit 31, with a leading end portion of the label piece 200 discharged to the discharge opening 105 of the apparatus body 100. In the present embodiment, the outer circumferential surface of the discharge roller 32 for holding the label piece 200 is treated, for example, with an anti-adhesive for preventing the adhesive surface 2 b of the label piece 200 from adhering to the surface of the discharge roller 32, to thereby make the adhesive surface 2 b of the label piece 200 separable from the surface of the discharge roller 32. In this case, the discharge roller 32 as a whole may be made of a material separable from the adhesive surface 2 b. The sheet sensor 34 detects the presence or absence of the print medium 2. The sheet sensor 34 may be a photosensor or a combination of a mechanical lever and a photosensor, for example.

In the present embodiment, the guiding device 106 is constructed of the facing roller 24, a second roller 42, a third roller 43, an endless guide belt 44, a holder 45, and a shaft 46. The facing roller 24, which serves as a component of the transporting unit 103, as described above, also serves as a component of the guiding device 106. The second roller 42 serving as a separation roller is disposed downstream of the facing roller 24 serving as a first roller and upstream of the image forming unit 102 in the medium transporting direction. The third roller 43 is disposed on the opposite side of the second roller 42 across the facing roller 24.

The guide belt 44 is stretched around the facing roller 24, the second roller 42, and the third roller 43. In the present embodiment, the guide belt 44 is a belt member including a base material made of polyimide and an outer circumferential surface formed with a release layer (e.g., a silicone coating layer) on the base material to improve the releasability of the guide belt 44 from the adhesive surface 2 b of the print medium 2.

The facing roller 24, the second roller 42, and the third roller 43 are rotatably held by the holder 45. The holder 45 is disposed to be rotatable about the shaft 46 to allow the facing roller 24 to move between a position at which the facing roller 24 faces the transport roller 22 and a position at which the facing roller 24 is separated from the transport roller 22 to open the space between the facing roller 24 and the transport roller 22. To install the roll 4 in the sheet feeding unit 101 and set the print medium 2 on the protection belt 21, the space between the facing roller 24 and the transport roller 22 is opened. To transport the print medium 2, the facing roller 24 is pressed against the transport roller 22. Therefore, the facing roller 24 is pressed against the transport roller 22 by a biasing device such as a spring. Similarly, the second roller 42 is also pressed against the protection belt 21 by a biasing device such as a spring.

As described above, the present embodiment is configured to perform image formation on the print medium 2 with the adhesive surface 2 b facing the protection belt 21. Alternatively, the image formation may be performed on the adhesive surface 2 b of the print medium 2. In this case, it is preferable that the outer circumferential surface of the guide belt 44 is treated with an anti-adhesive for preventing the adhesive surface 2 b of the print medium 2 from adhering to the surface of the guide belt 44.

In the thus-configured image forming apparatus, the protection belt 21 and the print medium 2 unwound from the roll 4 installed in the sheet feeding unit 101 are together clamped between the transport roller 22 and the facing roller 24. Then, the transport roller 22 is driven to rotate to transport the print medium 2 with the adhesive surface 2 b protected by the protection belt 21, and a desired image is formed on the print medium 2 by the recording heads 11 of the image forming unit 102. The print medium 2 having the image formed thereon is then separated from the protection belt 21 and sent to the sheet discharging unit 104 to be cut into the label piece 200 at a predetermined position by the cutter unit 31.

The label piece 200 is held between the discharge roller 32 and the spur roller group 33 to be dischargeable from the discharge opening 105 of the apparatus body 100.

Particularly in a case in which the image is formed on the adhesive surface 2 b of the print medium 2, the guiding device 106 prevents the print medium 2 from being caught in the facing roller 24 during the transport or rewinding of the print medium 2. Without the guide belt 44, the adhesive surface 2 b of the print medium 2 may stick to and be caught in the outer circumferential surface of the facing roller 24 due to the relatively small curvature of the facing roller 24, even if the outer circumferential surface of the facing roller 24 is treated with an anti-adhesive. Conceivably, the curvature of the facing roller 24 could be increased to prevent such a transport failure. The increase in curvature of the facing roller 24, however, reduces the area of a clamp region between the facing roller 24 and the transport roller 22, making it difficult to obtain stable transport performance.

In the present embodiment, therefore, the print medium 2 in the medium transporting operation is transported while being held by the guide belt 44, and is reliably separated from the guide belt 44 by the second roller 42 having a relatively large curvature and serving as a separation roller, thereby preventing the print medium 2 from being caught in the facing roller 24 in the medium transporting operation. Also in the rewinding of the print medium 2, the guide belt 44 receives the adhesive surface 2 b of the print medium 2 to prevent the print medium 2 from being caught in the facing roller 24.

After the image formation and the cutting of the print medium 2 by the cutter unit 31, a leading end portion of the print medium 2 is located at the position of the cutter unit 31. If the next image forming operation starts in this state, a portion of the print medium 2 facing the image forming unit 102 will be wasted without being used (i.e., with no image formed thereon). To prevent this, the print medium 2 is rewound in a rewinding direction opposite to the medium transporting direction to a position at which the leading end portion of the print medium 2 is located before (i.e., upstream of) the image forming unit 102 in the medium transporting direction.

With reference to FIGS. 4 to 8, description will now be given of details of the sheet feeding unit 101 (i.e., sheet feeder) according to the present embodiment. FIG. 4 is a perspective view of the sheet feeding unit 101, in which the roll 4 is installed. FIG. 5 is a perspective view of the sheet feeding unit 101, from which the roll 4 is removed. FIG. 6 is a front view of the sheet feeding unit 101 with the roll 4 installed. FIG. 7 is a front view of the sheet feeding unit 101 with the openably closable member 8 thereof opened. FIG. 8 is a front view of the sheet feeding unit 101 and the roll 4 removed therefrom. FIGS. 6 to 8 illustrate components such as the roll holder 6 in a see-through manner to clarify the relative positions of the components. In FIG. 6, arrow A indicates the direction in which the print medium 2 is unwound from the roll 4. In FIG. 8, arrow B indicates the direction in which the spool 5 carrying the roll 4 is installed into or removed from the roll holder 6.

As illustrated in FIG. 5, in the sheet feeding unit 101, the two roll holders 6 are disposed which rotatably hold opposed end portions of the spool 5 around which a core member of the roll 4 is fitted. In the present specification, the term “spool” is not limited to a member provided separately from the core member of the roll 4, and may refer to a member formed integrally with the core member of the roll 4 and configured to be held by the roll holders 6. If the core member of the roll 4 is directly held by the roll holders 6, such a core member serves as a spool.

As described above, each of the roll holders 6 includes a spool bearing 7, an openably closable member 8, a first roller 111, a second roller 112, a third roller 113, a shaft 9, a shaft 114, a roller holder 115, and a tension spring 116. The opposed end portions of the spool 5 are fitted in the spool bearings 7. Each of the spool bearings 7 includes a spool bearing portion 7 a and an engaging portion 7 b. The spool bearing portion 7 a receives the corresponding one of the end portions of the spool 5 fitted therein, and is opened and closed by the openably closable member 8 including a hook 8 a. The openably closable member 8 is rotatably attached to the spool bearing 7 by the shaft 9. When the hook 8 a of the openably closable member 8 is engaged with the engaging portion 7 b of the spool bearing 7, the spool bearing portion 7 a is placed in the closed state.

The first roller 111, the second roller 112, and the third roller 113 serving as first to third rotary members, respectively, contact the outer circumferential surface of the spool 5 to rotatably hold the spool 5. In place of the rollers, the rotary members may be spherical members. As viewed in the axial direction of the spool 5, the first roller 111 and the second roller 112 are fixedly disposed on one side of the spool 5 on which the print medium 2 is unwound from the roll 4, with the unwound print medium 2 located between the first roller 111 and the second roller 112, as illustrated in FIG. 6. That is, the first roller 111 and the second roller 112 are rotatably held by the spool bearing 7 and the openably closable member 8, respectively, and are fixed in position when holding the spool 5 carrying the roll 4. Herein, the term “fixed” refers to the state in which the first roller 111 and the second roller 112 are rotatable but immovable in a direction substantially perpendicular to the axial direction (except for movement due to play allowing rotation).

Meanwhile, the third roller 113 is disposed on another side of the spool 5 opposite to the position at which the print medium 2 is unwound from the roll 4. That is, the third roller 113 is rotatably held by one end portion of the roller holder 115 rotatably held by the spool bearing 7 via the shaft 114, to be displaceable toward the axis of the spool 5. Between the other end portion of the roller holder 115 and the spool bearing 7, the tension spring 116 is provided which serves as a biasing device that biases the third roller 113 located at the one end portion of the roller holder 115 in a direction of pressing the spool 5.

In the present embodiment, the roll 4 of the print medium 2 having the adhesive surface 2 b with no backing sheet is installed in the sheet feeding unit 101, and the print medium 2 is unwound from the roll 4 and clamped between the transport roller 22 and the facing roller 24 to be fed and transported. In this process, a relatively large peel force for peeling the print medium 2 off the roll 4 is generated owing to the adhesive force of the adhesive surface 2 b. In this case, if the spool 5 carrying the roll 4 is held by a two-point support bearing or a bearing having a substantially U-shaped groove, the relatively large peel force for peeling the adhesive surface 2 b acts on the roll 4, causing a shift in position of the spool 5. The shift in position of the spool 5 prevents smooth release of the print medium 2 from the roll 4, resulting in unstable transport of the print medium 2.

The present embodiment, therefore, supports the spool 5 at three points by using the first roller 111, the second roller 112, and the third roller 113, thereby allowing the spool 5 carrying the roll 4 to smoothly rotate without a shift in position. Accordingly, the print medium 2 is stably transported despite the relatively large peel force acting on the roll 4.

Further, as viewed in the axial direction of the spool 5, the roll holder 6 is configured such that the rotatable first and second rollers 111 and 112, which are fixedly disposed to sandwich the print medium 2 unwound from the roll 4, hold one side of the spool 5 on which the print medium 2 is unwound from the roll 4, and that the rotatable and movable third roller 113 presses and holds another side of the spool 5 opposite to the position at which the print medium 2 is unwound.

In the configuration supporting the spool 5 at three points, it is desirable that one of the three points movably presses and holds the spool 5 to smoothly rotate the spool 5 while absorbing variation in diameter of the spool 5. If the point of pressing and holding the spool 5 is located on one side of the spool 5 on which the print medium 2 is unwound, and if the peel force is relatively large, the force for pulling the roll 4 may exceed the force for pressing the spool 5, resulting in a shift in position of the spool 5. To prevent the spool 5 from moving in the unwinding direction despite the relatively large peel force, therefore, the first roller 111 and the second roller 112 are fixedly disposed on the side of the spool 5 on which the print medium 2 is unwound from the roll 4, with the unwound print medium 2 located between the first roller 111 and the second roller 112. Consequently, a stable transporting operation is obtained, and the image quality is improved. Further, the third roller 113 holding another side of the spool 5 opposite to the position of unwinding the print medium 2 is pressed against the spool 5, thereby eliminating backlash when the roll 4 is rotated to feed the print medium 2, and thus providing a more stable transporting operation.

With reference to FIG. 9, description will now be given of a drive force transmission mechanism 130 that transmits drive force from a drive source (riot illustrated) in the apparatus body 100 to the spool 5 holding the roll 4. FIG. 9 is a front view of the sheet feeding unit 101 similar to that of FIG. 6. In FIG. 9, arrow A indicates the direction in which the print medium 2 is unwound from the roll 4, arrow C indicates the direction in which the spool 5 is displaced, and arrow D indicates the direction in which gears 131 and 134 mesh with each other.

If the roll 4 is a bonded roll having a core member bonded to an end portion of the print medium 2, or if the transporting unit 103 and other units in the apparatus body 100 are integrated as one unit drawable from the apparatus body 100, for example, the print medium 2 transported onto the protection belt 21 of the transporting unit 103 is rewound onto the roll 4 as necessary.

In the present image forming apparatus, therefore, at least rotational drive force from the drive source for rotating the transport roller 22 in the direction of rewinding the print medium 2 onto the roll 4 is transmitted to the spool 5 via the drive force transmission mechanism 130. The drive force transmission mechanism 130 includes gears 131, 132, 133, and 134 and other gears forming a gear train and a timing belt (not illustrated). In the present embodiment, the drive force transmission mechanism 130 transmits, via the gear train including the gears 131, 132, 133, and 134 and the timing belt, the rotary motion of the transport roller 22 to the spool 5 connected to the gear 131. In this process, the gear 134 in mesh with the gear 131 connected to the spool 5 is disposed at a position at which the gear 134 meshes with the gear 131 in a direction substantially perpendicular to the direction of unwinding the print medium 2.

With this configuration, even if the spool 5 is slightly displaced by the play of the first and second rollers 111 and 112 in the direction of arrow C in accordance with the operation of feeding and transporting the print medium 2, the mesh between the gears 131 and 134 is maintained. The configuration therefore prevents the gear 134 from biting into the gear 131 and causing a rotation failure or an increase in rotational load. Conversely, if the gear 134 meshes with the gear 131 in, for example, a direction facing the displacement direction of the spool 5, the gear 131 bites into the gear 134 in accordance with the displacement of the spool 5, causing a rotation failure or an increase in rotational load. As a result, a transport load is increased during the transport of the print medium 2, preventing stable transport of the print medium 2 and degrading the transport accuracy.

According to the above-described embodiment of the present invention, the accuracy of transporting a linerless label sheet is improved.

In the present specification, the term “image formation” refers to providing a medium with a meaningful image such as a character or a figure and also providing a medium with a meaningless image such as a pattern (i.e., simple ejection of liquid droplets onto a medium). Further, the term “ink” is not limited to so-called ink, and is used to collectively refer to various types of liquids with which the image formation is performed, such as recording liquid and fixing liquid. Further, the image forming apparatus includes both a serial-type image forming apparatus and a line-type image forming apparatus.

The above-described embodiments and effects thereof are illustrative only and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements or features of different illustrative embodiments herein may be combined with or substituted for each other within the scope of this disclosure and the appended claims. Further, features of components of the embodiments, such as number, position, and shape, are not limited to those of the disclosed embodiments and thus may be set as preferred. It is therefore to be understood that, within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein. 

What is claimed is:
 1. A sheet feeder comprising: a spool configured to hold a roll of print medium having an adhesive surface with no release liner attached thereto, with the roll fitted around the spool; and a pair of roll holders configured to hold the spool at opposed ends of the spool, each including at least a first rotary member, a second rotary member, and a third rotary member that contact an outer circumferential surface of the spool at different locations along the outer circumferential surface of the spool to rotatably hold the spool.
 2. The sheet feeder according to claim 1, wherein the first and second rotary members are fixedly disposed on one side of the spool on which the print medium is unwound from the roll, with the unwound print medium located between the first and second rotary members, and the third rotary member is displaceably disposed on another side of the spool opposite to a position at which the print medium is unwound from the roll.
 3. The sheet feeder according to claim 2, further comprising: a biasing device configured to press the third rotary member against the spool.
 4. The sheet feeder according to claim 1, wherein each of the roll holders includes an openably closable member configured to open and close relative to the roll holder, and one of the first and second rotary members is provided to the openably closable member.
 5. An image forming apparatus comprising: a sheet feeder including a spool configured to hold a roll of print medium having an adhesive surface with no release liner attached thereto, with the roll fitted around the spool, and a pair of roll holders configured to hold the spool at opposed ends of the spool, each including at least a first rotary member, a second rotary member, and a third rotary member that contact an outer circumferential surface of the spool at different locations along the outer circumferential surface of the spool to rotatably hold the spool; and an image forming device configured to form an image on the print medium fed from the roll by the sheet feeder.
 6. The image forming apparatus according to claim 5, wherein the first and second rotary members are fixedly disposed on one side of the spool on which the print medium is unwound from the roll, with the unwound print medium located between the first and second rotary members, and the third rotary member is displaceably disposed on another side of the spool opposite to a position at which the print medium is unwound from the roll.
 7. The image forming apparatus according to claim 6, further comprising: a biasing device configured to press the third rotary member against the spool.
 8. The image forming apparatus according to claim 5, wherein each of the roll holders includes an openably closable member configured to open and close relative to the roll holder, and one of the first and second rotary members is provided to the openably closable member. 